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                <h1 id="firstHeading" class="firstHeading">constexpr specifier <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<dl><dd>
<ul><li>
<code><b>constexpr</b></code> - specifies that the value of a variable or function can appear in constant expressions
</li></ul>
</dd></dl>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span>
</h3>
<p>The <code>constexpr</code> specifier declares that it is possible to evaluate the value of the function or variable at compile time. Such variables and functions can then be used where only compile time <a href="constant_expression.html" title="cpp/language/constant expression">constant expressions</a> are allowed. A <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> specifier used in an object declaration implies <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span>. A <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> specifier used in an function declaration implies <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span>.
</p>
<p>A <b><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> variable</b> must satisfy the following requirements:
</p>
<dl><dd>
<ul>
<li> its type must be a <a href="../concept/LiteralType.html" title="cpp/concept/LiteralType"><code>LiteralType</code></a>.
</li>
<li> it must be immediately constructed or assigned a value.
</li>
<li> the constructor parameters or the value to be assigned must contain only literal values, <code>constexpr</code> variables and functions.
</li>
<li> the constructor used to construct the object (either implicit or explicit) must satisfy the requirements of <code>constexpr</code> constructor. In the case of explicit constructor, it must have <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> specified. 
</li>
</ul>
</dd></dl>
<p>A <b><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> function</b> must satisfy the following requirements:
</p>
<dl><dd>
<ul>
<li> it must not be virtual
</li>
<li> its return type must be <a href="../concept/LiteralType.html" title="cpp/concept/LiteralType"><code>LiteralType</code></a>
</li>
<li> each of its parameters must be literal type
</li>
</ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx14">
<td>
<dl><dd>
<ul><li> the function body must be either deleted or defaulted or contain only the following:
</li></ul>
<dl><dd>
<ul>
<li> null statements
</li>
<li> <a href="static_assert.html" title="cpp/language/static assert">static_assert</a> declarations
</li>
<li> <a href="typedef.html" title="cpp/language/typedef">typedef</a> declarations and <a href="type_alias.html" title="cpp/language/type alias">alias</a> declarations that do not define classes or enumerations
</li>
<li> using declarations
</li>
<li> using directives
</li>
<li> exactly one <a href="return.html" title="cpp/language/return">return</a> statement that contains only literal values, <code>constexpr</code> variables and functions.
</li>
</ul>
</dd></dl>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx14">(until C++14)</span></td>
</tr>
<tr class="t-rev t-since-cxx14">
<td>
<dl><dd>
<ul><li> the function body must be either deleted or defaulted or contain <b>any statements except</b>:
</li></ul>
<dl><dd>
<ul>
<li> an <a href="asm.html" title="cpp/language/asm">asm declaration</a>
</li>
<li> a <a href="goto.html" title="cpp/language/goto">goto</a> statement
</li>
<li> a <a href="try_catch.html" title="cpp/language/try catch">try-block</a>
</li>
<li> a definition of a variable of non-literal type or of static or thread <a href="storage_duration.html" title="cpp/language/storage duration">storage duration</a> or for which no initialization is performed.
</li>
</ul>
</dd></dl>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
</tr>
</table>
<p>A <b><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> constructor</b> must satisfy the following requirements:
</p>
<dl><dd>
<ul>
<li> each of its parameters must be literal type
</li>
<li> the class must have no virtual base classes
</li>
<li> the constructor must not have a <a href="function-try-block.html" title="cpp/language/function-try-block">function-try-block</a>
</li>
</ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx14">
<td>
<dl><dd>
<ul><li> the constructor body must be either deleted or defaulted or contain only the following:
</li></ul>
<dl><dd>
<ul>
<li> null statements
</li>
<li> <a href="static_assert.html" title="cpp/language/static assert">static_assert</a> declarations
</li>
<li> <a href="typedef.html" title="cpp/language/typedef">typedef</a> declarations and <a href="type_alias.html" title="cpp/language/type alias">alias</a> declarations that do not define classes or enumerations
</li>
<li> using declarations
</li>
<li> using directives
</li>
</ul>
</dd></dl>
<ul><li> every base class and every non-static member must be initialized, either in the constructors initialization list or by a member brace-or-equal initializer. In addition, every constructor involved must be a constexpr constructor and every clause of every brace-or-equal initializer must be a constant expression
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx14">(until C++14)</span></td>
</tr>
<tr class="t-rev t-since-cxx14">
<td>
<dl><dd>
<ul><li> the constructor body must be either deleted or defaulted or satisfy the following constraints:
</li></ul>
<dl><dd>
<ul>
<li> the compound statement of the constructor body must satisfy the constraints for the body of a constexpr function
</li>
<li> for the constructor of a <a href="class.html" title="cpp/language/class">class or struct</a>, every base class sub-object and every <a href="union.html%23Union-like_classes" title="cpp/language/union">non-variant</a> non-static data member must be initialized. If the class is a <a href="union.html%23Union-like_classes" title="cpp/language/union">union-like class</a>, for each of its non-empty anonymous union members, exactly one variant member must be initialized
</li>
<li> for the constructor of a non-empty <a href="union.html" title="cpp/language/union">union</a>, exactly one non-static data member must be initialized
</li>
<li> every constructor involved in initializing non-static members and base class must be a constexpr constructor.
</li>
</ul>
</dd></dl>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
</tr>
</table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span>
</h3>
<p><a href="../keyword/constexpr.html" title="cpp/keyword/constexpr"><tt>constexpr</tt></a>
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span>
</h3>
<div class="t-example">
<p>
</p>
<p>Definition of a constexpr function which computes factorials and a literal type that extends string literals:
</p>
 <div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;stdexcept&gt;</span>
 
<span class="co1">// constexpr functions use recursion rather than iteration</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> factorial<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> n <span class="sy1">&lt;=</span> <span class="nu0">1</span> <span class="sy4">?</span> <span class="nu0">1</span> <span class="sy4">:</span> <span class="br0">(</span>n <span class="sy2">*</span> factorial<span class="br0">(</span>n<span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// literal class</span>
<span class="kw1">class</span> conststr <span class="br0">{</span>
    <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span> p<span class="sy4">;</span>
    <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> sz<span class="sy4">;</span>
 <span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">&gt;</span>
    <span class="kw4">constexpr</span> conststr<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="br0">(</span><span class="sy3">&amp;</span>a<span class="br0">)</span><span class="br0">[</span>N<span class="br0">]</span><span class="br0">)</span> <span class="sy4">:</span> p<span class="br0">(</span>a<span class="br0">)</span>, sz<span class="br0">(</span>N<span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    <span class="co1">// constexpr functions signal errors by throwing exceptions from operator ?:</span>
    <span class="kw4">constexpr</span> <span class="kw4">char</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
        <span class="kw1">return</span> n <span class="sy1">&lt;</span> sz <span class="sy4">?</span> p<span class="br0">[</span>n<span class="br0">]</span> <span class="sy4">:</span> <span class="kw1">throw</span> <a href="../error/out_of_range/out_of_range.html"><span class="kw614">std::<span class="me2">out_of_range</span></span></a><span class="br0">(</span><span class="st0">""</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span>
    <span class="kw4">constexpr</span> <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> sz<span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">constexpr</span> <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> countlower<span class="br0">(</span>conststr s, <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <span class="sy1">=</span> <span class="nu0">0</span>,
                                             <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> c <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">return</span> n <span class="sy1">==</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy4">?</span> c <span class="sy4">:</span>
           s<span class="br0">[</span>n<span class="br0">]</span> <span class="sy1">&gt;=</span> <span class="st0">'a'</span> <span class="sy3">&amp;&amp;</span> s<span class="br0">[</span>n<span class="br0">]</span> <span class="sy1">&lt;=</span> <span class="st0">'z'</span> <span class="sy4">?</span> countlower<span class="br0">(</span>s, n<span class="sy2">+</span><span class="nu0">1</span>, c<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy4">:</span>
           countlower<span class="br0">(</span>s, n<span class="sy2">+</span><span class="nu0">1</span>, c<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// output function that requires a compile-time constant, for testing</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> n<span class="sy1">&gt;</span> <span class="kw1">struct</span> constN <span class="br0">{</span>
    constN<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"4! = "</span> <span class="sy4">;</span>
    constN<span class="sy1">&lt;</span>factorial<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy1">&gt;</span> out1<span class="sy4">;</span> <span class="co1">// computed at compile time</span>
 
    <span class="kw4">volatile</span> <span class="kw4">int</span> k <span class="sy1">=</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="co1">// disallow optimization using volatile</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> k <span class="sy1">&lt;&lt;</span> <span class="st0">"! = "</span> <span class="sy1">&lt;&lt;</span> factorial<span class="br0">(</span>k<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// computed at run time</span>
 
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Number of lowercase letters in <span class="es1">\"</span>Hello, world!<span class="es1">\"</span> is "</span><span class="sy4">;</span>
    constN<span class="sy1">&lt;</span>countlower<span class="br0">(</span><span class="st0">"Hello, world!"</span><span class="br0">)</span><span class="sy1">&gt;</span> out2<span class="sy4">;</span> <span class="co1">// implicitly converted to conststr</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">4! = 24
8! = 40320
Number of lowercase letters in "Hello, world!" is 9</pre></div></div> 
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